It was first reported by M. Poulain and J. Lucas in "Verres Fluores au Tetrafluorure de Zirconium. Properties Optiques d'un Verre Dope au Nd.sup.3+ ", Mat. Res, Bull. 10, 243-246 (1975) that a transparent glass was formed from a ZrF.sub.4.BaF.sub.2.NaF mixture fused at 800.degree. C., rather than a new crystalline laser host material, which had been the object of their research. Their finding aroused immediate widespread attention because this previously unknown fluoride-based glass system offered the only practical amorphous material with infrared transparency extending beyond 6 microns. This desirable optical property is founded in the nonoxide composition of the glass and has comprised the principal basis for extensive research and development to the present day.
Investigators in the field appreciated that, because of the extended infrared transparency exhibited by the fluoride, they had the potential of forming optical fiber waveguides with losses of one to two orders of magnitude less than silica fibers. Furthermore, the low energy phonon spectra lead to relatively high quantum efficiencies for many rare earth metal transitions. Finally, when formulated with a sufficient content of rare earth metal ions, the glasses hold the promise of being attractive hosts for active fibers.
Among the more potentially useful of the fluoride-based glasses is the system of heavy metal fluoride glasses termed ZBLAN, those glasses consisting essentially of ZrF.sub.4, BaF.sub.2, LaF.sub.3, AlF.sub.3, and NaF. U.S. Pat. No. 4,674,835 (Mimura et al.) recites a representative formulation of ZBLAN expressed in terms of mole percent
______________________________________ ZrF.sub.4 50-55 AlF.sub.3 2-4 BaF.sub.3 16-24 NaF 16-24 LaF.sub.3 3-5 ______________________________________
wherein the sum of the components totals 100.
Unfortunately, heavy metal fluoride glasses suffer certain undesirable attributes which have restricted their applications. Most notably, heavy metal fluoride glasses exhibit poor resistance to devitrification and Mimura et al. discuss the crystallization problems of ZBLAN and the light scattering problems resulting therefrom.
The great susceptibility of ZBLAN glasses to devitrification also generates problems in forming large preforms. Crystallization at the interface between the core and cladding during the production of the preform causes problems in the most commonly used methods for preparing a ZBLAN optical fiber. That is, heavy metal fluoride glasses are quite prone to inhomogeneous nucleation, the consequence of which being crystallization at the core and cladding interfaces, particularly during the drawing of the optical fiber. The resulting fibers are subject to serious scattering losses due to crystals in the fibers.
Devitrification of the glasses is aggravated when ions necessary to impart differences in indices of refraction to the core and cladding are added to the glass composition. Additional doping, for example with rare earth metal ions, also tends to reduce the stability of the glass. As a consequence of those problems, research has been continuous to find additives to the base ZBLAN composition which will reduce the tendency of the glass to devitrify and to increase the chemical stability thereof. Because that research has ameliorated the devitrification problem to a certain extent, but has not nearly eliminated it, other composition areas have been investigated wherein properties similar to those demonstrated by ZBLAN glasses can be secured, but where the problem of devitrification would be significantly reduced, most preferably totally eliminated.
One area of such research is reported by Y. Wang and J. Ohwaki in "New Transparent Vitroceramics Codoped with Er.sup.3+ and Yb.sup.3+ for Efficient Frequency Upconversion", Applied Physics Letters, 63 (24), 3268-3270, Dec. 13, 1993. The specific vitroceramic (also called glass-ceramic) described therein had a base composition within the general fluoroaluminosilicate system and consisted essentially, expressed in terms of mole percent, of
______________________________________ SiO.sub.2 30 CdF.sub.2 20 AlO.sub.1.5 15 YbF.sub.3 10 PbF.sub.2 24 ErF.sub.3 1. ______________________________________
The glass produced from that composition was heat treated at 470.degree. C. to develop microcrystallites therewithin identified as Pb.sub.x Cd.sub.1-x F.sub.2, which the authors stated did not reduce the transparency of the body. The authors posited that the Yb.sup.3+ and Er.sup.3+ ions were preferentially segregated from the precursor glass and dissolved into the microcrystals upon heat treatment. The size of the microcrystallites was estimated by the authors to range about 20 nm (200 .ANG., 002 .mu.m); that size being so small that light scattering loss was minimal. (Our investigations of the materials indicated that not only were the crystals of very small size, but also the interparticle spacing of the crystals was very small.) The authors reported the upconversion efficiency of their products to be about 2 to 10 times as high as that measured on the precursor glass and other fluoride-containing glasses.
Based upon those findings by the authors, we hypothesized that, if those glass-ceramics were as transparent as described, they might also be useful as hosts in amplifier and/or laser devices. Nevertheless, we realized that, if the glass-ceramic materials were to comprise viable hosts for 1.3 .mu.m amplifier devices, Yb would have to be eliminated from the composition because Pr, which is customarily utilized in materials designed for such devices, readily transfers electrons to Yb, that action resulting in the efficiency of the device being reduced.
Therefore, the principal objective of the present invention was to devise glass-ceramic materials comprising an improvement upon those described by Wang and Ohwaki.
A specific objective of the subject invention was to develop such glass-ceramic materials which, when doped with Pr, would exhibit excellent behavior as hosts in 1.3 .mu.m amplifiers.
Another specific objective of the instant invention was to develop optical fiber waveguides comprising said glass-ceramic materials as the high refractive index core surrounded by a cladding of lower refractive index material.